Before your kid starts any task that takes concentration, turn off all gadgets, including televisions, iPads, and cell phones. If they need to go on a computer for homework, set it up for them in advance so they're not tempted to use it for anything but studying.
If you have other children in the home who are playing or watching iPads or TV, make sure the child who needs to focus is far enough away that they won't be distracted. In addition, make sure that everything your child needs to complete their assignment is close at hand, so they don't have to stop and get up to get materials or even a drink of water.